The DOBA is a high acid crude oil originating from the Chad region of West Africa. The DOBA is known to contain calcium naphthenate and the amount of calcium naphthenate varies over a range from about 150 to about 700 ppm. In a typically supplied DOBA crude oil, the amount of calcium naphthenate may vary from about 250-about 300 ppm.
The DOBA is a heavy high acid crude oil with Total Acid Number [TAN] ranging upwards of 4.0 mg KOH/gm of sample and the API gravity is about 19. The sulfur content in DOBA is very low to nil.
The DOBA is typically a crude oil with a lot of residue in it and for proper blending, typically internationally, refiners blend it with very light crude oil or condensates to increase the API of the resulted blend to more than 30. Such blending with light crude oil or condensates helps to create sufficient light ends to help achieve product yields for the crude distillation unit. The most of light crude oil or condensates thus selected generally have very little to nil sulfur contents, which means the overall sulfur content still remains very low. Further, the hydrogen sulfide (H2S) being oil soluble is not present in relatively higher quantities in these types of blends.
The pH of the wash water for desalter is generally adjusted by addition of or due to presence of alkaline medium, selected from the group comprising sodium hydroxide (NaOH or caustic), ammonia or amine compound, or mixture thereof. The pH of the wash water (before mixing with crude oil) or of the wash water in the desalter generally varies from 5 to 11 pH, preferably from 5 to 9 pH, more preferably from 6 to 9 pH, even more preferably from 7 to 9 pH.
The inventor of present invention has observed that if solution of calcium naphthenate in an organic solvent, for example toluene having concentration of Ca of about 2247 ppm is treated with equal weight of water by heating to about 130 degree C. in a Parr autoclave under autogenous pressure, and separated into organic and aqueous layers in a separating funnel, no black layer is formed at the interface in presence of water. When organic layer, as separated, was dried by evaporating toluene, its acid value was found to be very low of about 48.36 (mg KOH per gm). The low Acid Value indicates that calcium naphthenate does not hydrolyze appreciably merely in presence of water.
The inventor of present invention has further observed that when DOBA or its blend containing calcium naphthenate is treated with additives as known in the art, for example with glycolic acid, malic acid, citric acid, maleic anhydride, benzaldehyde (aromatic aldehyde) and glutaraldehyde (aliphatic di-aldehyde) it does not hamper removal of metals including calcium from such DOBA crude oil or its blend at low pH, i.e. at the pH after addition of the selected additive in the wash water, but without adjusting it with addition of alkaline medium.
The inventor has, experimentally, found that the efficiency of glycolic acid to remove calcium from crude oils containing calcium naphthenate is low, i.e. 79.3% at the pH of about 2.52 of the wash water containing the glycolic acid, which, surprisingly and unexpectedly, substantially reduces further to about 23.7% to 21%, to about 52.3% to 36.5%, to about 56.30% to 51.9% if the pH of the wash water or wash water in the desalter is increased to 5 or above 5 to respectively due to the presence of sodium hydroxide, ammonia and monoethanolamine (MEA) in the wash water or wash water in the desalter.
The inventor has, experimentally, also found that the efficiency of malic acid to remove calcium from crude oils containing calcium naphthenate is low, i.e. 83.6% at the pH of about 2.3 of the wash water containing the malic acid, which, surprisingly and unexpectedly, substantially reduces further to about 24.3% to 15%, to about 54.2% to 45.7%, to about 73.4% to 61.9% if the pH of the wash water or wash water in the desalter is increased to 5 or above 5 to 9 respectively due to the presence of sodium hydroxide, ammonia and monoethanolamine in the wash water or wash water in the desalter.
The inventor has, experimentally, also found that the efficiency of citric acid to remove calcium from crude oils containing calcium naphthenate is low, i.e. 78.2% at the pH of about 2 to 3 of the wash water containing the citric acid, which, surprisingly and unexpectedly, substantially reduces further to about 42.3% to 17%, to about 60.4% to 56.3% if the pH of the wash water or wash water in the desalter is increased to 5 or above 5 to 9 respectively due to the presence of sodium hydroxide and ammonia in the wash water or wash water in the desalter.
Furthermore, the inventor has observed that when malic acid or citric acid is employed, they form precipitates, and hence, their use also suffer from problems of fouling of the equipments.
The inventor has, experimentally, also found that the efficiency of maleic anhydride to remove calcium from crude oils containing calcium naphthenate is low, i.e. 83.5% at the pH of about 2 to 3 of the wash water containing the maleic anhydride, which, surprisingly and unexpectedly, substantially reduces further to about 43.9% to 15%, to about 53.0% to 41.3%, to about 73.3% to 51.4% if the pH of the wash water or wash water in the desalter is increased to 5 or above 5 to 9 respectively due to the presence of sodium hydroxide, ammonia and monoethanolamine in the wash water or wash water in the desalter.
The inventor has, experimentally, also found that the efficiency of benzaldehyde to remove calcium from crude oils containing calcium naphthenate is very low, i.e. 20.3% at the pH of about 3.4 of the wash water containing the benzaldehyde, which, itself being very low, no further experiments were conducted to see effect of alkaline media generally present in the wash water.
The inventor has, experimentally, also found that the efficiency of glutaraldehyde to remove calcium from crude oils containing calcium naphthenate is very low, i.e. 35.9% at the pH of about 4.2 of the wash water containing the glutaraldehyde, which, itself being very low, no further experiments were conducted to see effect of alkaline media generally present in the wash water.
Accordingly, the industry processing DOBA or its blends containing calcium naphthenate faces serious problems in removing calcium from such oils or their blends, particularly when pH of wash water for desalter or of the wash water in the desalter varies between about 5 to about 11, particularly varies between about 6 to about 11, more particularly varies between about 7 to about 9.